RJ-45 Serial: A Tale in Confusion

21st Thursday, 2012  |  Engineering News, Life at LAVA  |  no comments

It’s a common problem we see at LAVA — customers calling up with cabling issues. The most problematic are RJ-45 connectors providing RS-232 interfaces, since there is no solidly standardized pin-out. This issue of RJ-45 to RS-232 connection crops up frequently in POS (Point of Sale) systems, and shows up in other hardware as well.

A recent call to our support lines exemplifies the problem. A customer called up with a question: why would the LAVA Ether-Serial Link 4-232-RJ45 that he wanted to connect to a Valcom 2924 intercom system connect with a null modem cable, but not with a straight-through cable? On the face of it, the question seems nonsensical: evidently, if one cable works, then the other should not. Where was the problem? The LAVA Ether-Serial Link is configured as a Data Terminal Equipment (DTE) device, and since a null modem cable worked, the Valcom 2924 was also a DTE, so the connection was DTE-to-DTE. (If a straight-through cable had worked, then the intercom system would be a piece of Data Circuit-terminating Equipment (DCE), and the connection would be DTE-to-DCE).

Setting aside the fact that the term “null modem cable” can mean a number of things (see our blog post on null modem or crossover cables), you would think that ANY null modem cable, should it work, would not be replaceable with a straight-through cable. Common sense, right? The only problem was that the customer said the setup HAD worked with BOTH types of cables, when he was using a now-discontinued serial device server that was not a product of LAVA’s.

A bit of digging into this revealed that the intercom system in question had a DB-9 RS-232 printer port that could also be a source of Station Message Detail Recording (SMDR) information. Simple enough, but if the the intercom had a DB-9 serial port, why was the customer using a LAVA Ether-Serial Link with RJ-45 serial connectors? Why not just DB-9 to DB-9 and be done with it?

Valcom 2924

Valcom 2924

The answer lay with the discontinued serial device server the customer was wanting to replace — it was an Equinox ESP8. This serial device server has eight RS-232 interface ports, with RJ-45 connectors.

Equinox ESP8

Equinox ESP8

Now here is where it gets a bit funky. A look at the documentation for this device server revealed that Equinox was calling it a “Multi-Interface Serial Hub”, and available with it were a bunch of modular adapters and modular cables:

manual page

Modular DTE and DCE connectors

As you can see, the modular adapters make it possible to operate the ESP8 as either a DTE or DCE interface. There is a ten-wire “reversing cable” here — a serial cable to pair with the serial device server to be either a DCE or DTE device, as needed. This makes the device more versatile, in the sense that the cable is able to be either a straight-through or a null modem, WHEN USED WITH THIS DEVICE AND ITS ADAPTERS FROM EQUINOX. But it does nothing special for the LAVA Ether-Serial Link, which uses the tenth wire in an RS-232 RJ-45 connection for a totally different purpose: that is, as a way to supply power to serial peripherals over the serial connection.

manual page

Reversing Cable

See what I mean about the complications of RS-232 on an RJ-45 connector? And this is just the tip of the iceberg. Take a look at the multiplicity of arrangements of serial port lines on RJ-45 connectors documented on the following pages:

http://www.conserver.com/consoles/Signals/signals.html

http://www.opengear.com/alternate.html

So the bottom line really is: when setting up serial interfaces with RJ-45 connections, make no assumptions about what wire is running where or what cable will work, because odds are you will be wrong! Fully documented pinouts are essential for quick, error-free setups.